Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal backward curvature of the spine), excess lordosis (abnormal forward curvature of the spine), spondylolisthesis (forward displacement of a lumbar vertebra) and other disorders, such as ruptured or slipped discs, broken or fractured vertebrae and the like. Patients who suffer from such conditions usually experience extreme and debilitating pain. A technique known as spinal fixation uses surgical implants which mechanically immobilize areas of the spine assisting the eventual fusion of the treated vertebrae. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to bring to the patient relief from pain.
One particular technique for spinal fixation includes the immobilization of the spine by the use of a pair of spine rods that run generally parallel to the spine. In practicing this technique, bone screws are first fastened to the pedicles of the appropriate vertebrae or to the sacrum and act as the anchor points for the spine rods. The bone screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process. Clamp assemblies join the spine rods to the screws. The spine rods are generally custom-bent to achieve the desired curvature of the spinal column. Examples of such spinal fixation devices can be found in U.S. Pat. Nos. 4,653,481 and 5,030,220, which are incorporated herein by reference. For some applications, rather than using bone screws, lamina hooks can be fastened to a spine rod to connect the rod to a vertebra for distraction or compression.
It has been found that when a pair of spine rods are fastened in parallel on either side of the spinous process, the assembly can be significantly strengthened by using at least one additional rod to horizontally bridge the pair of spine rods. One such cross brace assembly is known as the dynamic transverse traction assembly (DTT), and is used with fixation devices employing a pair of spine rods. A cross brace assembly very similar to the DTT is disclosed in U.S. Pat. No. 5,084,049. Devices such as these commonly consist of a threaded rod for providing the desired lateral support. The threaded rod is fastened to each of the spine rods by clamps located on each end of the threaded rod. These devices, while providing good lateral support, are generally bulky due to the fact that the transverse rod is placed above the plane containing the spine rods to be bridged. This bulkiness can lead to complications in that it can later cause irritation of the patient's back muscles and other tissue which might rub against the device. A cross brace assembly that fits closer to the spine, preferably in the same general plane as the vertical spine rods, would reduce the complications associated with bulkier devices.
In recognition of the complications associated with bulky spinal fixation devices, new systems generally known as "low profile" spinal fixation systems have been developed. A low profile branch connector is desirable for use with such low profile systems.
There is also a growing trend toward the use of spinal fixation devices in the upper regions of the spine, namely for fixation of the thoracic and cervical vertebrae. However, there are fundamental differences in the physiology and structure of the vertebrae in these regions of the spine compared to the lumbar and sacral regions. Because of these differences, the spinal fixation devices are generally designed differently from a device used in the lumbar and sacral regions. One primary difference is that a spinal fixation device for the cervical region of the spine must be smaller than one for the lumbar or sacral region. It is desirable that branch connectors such as branch clamps and transverse clamps be versatile enough that their size could be scaled down to accommodate these smaller spinal fixation systems.
It has also been found that when a spinal fixation device is to extend along several vertebrae, for example, a device fastened to the sacrum and extending up through the lumbar vertebrae, a significant amount of bending of the rod is required in order to properly seat it in the screw-clamps while still achieving the desired curvature of the spine. Such bending, while necessary, is generally undesirable not only because bending the rod to an intricate curve can be difficult, but also because sharp bends in the rod can cause a structural weakening of the rod. Furthermore, a spine rod that extends along a large portion of the spine can be difficult to securely fasten to each of several screw-clamps or lamina hooks.
It is desirable to ease the assembly of a spinal fixation device by using a pair of shorter segments of spine rods that can first be fastened to the appropriate portions of the spine and then joined together by a branch connector. Furthermore, a branch connector capable of connecting one size of spine rod to a different size spine rod is also desirable. Such a device would be useful in a long spinal fixation device that spans from, for example, the thoracic region of the spine through the cervical region. In such a device, different size rods could be used for the different regions of the spine, yet could still be fastened securely together.
While clamp assemblies for fastening a pair of spine rods to a single bone screw are known, such clamp assemblies are difficult to work with as the location of the clamp assembly is limited by the placement of the bone screw which must be located at a vertebra, preferably at a pedicle. A clamping assembly that connects a pair of spine rods independently of a bone screw is desirable.
Branch connectors are also useful in a spine fixation system that uses spine rods along with both screw-clamps and hooks. In a system that includes screw-clamps, rods are generally positioned more laterally from the centerline of the spine than the more medially positioned rods in a system that includes lamina hooks. A branch connector can be used to reduce the amount of rod bending that is required when switching from a lateral rod placement along one portion of the spine to a medial rod placement along another portion of the spine.
Any clamps used for joining spine rods should be designed for high strength, yet fast and simple installation. As the installation of a spinal fixation device entails a lengthy and complicated surgical procedure it should be recognized than any simplification of the assembly procedure of a spinal fixation device will tend to reduce the complications associated with the procedure.